1. Field of the Invention
This invention relates to a method of and apparatus for coating a cylinder (particularly but not exclusively the wiping cylinder of an intaglio printing machine) with a fluid material (e.g. a heat-hardening plastics material) which hardens to a resilient coating.
2. Background Art
In an intaglio printing machine the wiping cylinders wipe and clean the engraved printing plate or cylinder. The wiping action simultaneously presses the printing ink into the etched detail of the engraving and cleans excess ink from the unengraved areas. In the remainder of each revolution of the wiping cylinder, its surface is washed so that it is completely clean before its next contact with the printing plate. For efficient wiping, therefore, the surface of the wiping cylinder has to be physically and chemically resilient enough to withstand continuous cleaning, and also the friction suffered during pressure against printing plates, and the attrition caused by abrasive pigments in printing inks.
It has been proposed to make such a wiping cylinder with a layer of a synthetic resin plastics composition (U.S. Pat. No. 4,054,685) by rotating the cylinder downwardly past a straight-edged scraper blade which extends parallel to the cylinder axis. The plastics material is supplied to the blade so that a thin layer is spread onto the cylinder. After the layer has been applied to the cylinder, it is heated and then allowed to cool, whereupon another layer can be applied to the underlying hardened layer.
Such an application method has a number of disadvantages.
First, it is a batch process involving separate layer application steps and layer heating steps. The effect of this separation is to lengthen substantially the time necessary for completion of the full coating process.
Second, the heating and coating steps are performed sequentially. Thus, the cylinder is cooling during coating, rather than staying at the optimum coating temperature.
Third, the final coating structure is a laminated one which can suffer from poor inter-layer adhesion, particularly when subject to the shear and torsional stresses of the wiping action to which the cylinder is subject in use.
Fourth, post-application heating of the full thickness of each applied layer can result in excess heating of the surface of the layer, in order to heat through the bulk of the layer. This can give rise to heterogeneity within the layer.